MakeWalk fails to retarget an animation with the rigify armature PitchiPoy Version
I had a similar issue with a ManuelbastionLab rig. The problem is due to the armature having bones that cannot be mapped by MakeWalk.

To solve it I moved the extra bones (in this case the breast bones) onto another layer.
To do this go into edit mode select the bones to move, pressM and select a layer that is invisible (this can be checked in the data tab). 



The light grey layer with the dot will not be visible.

Then I imported the BVH without any trouble.